py/compile: Fix edge case when constant-folding negation of integer.

Also adds tests specifically for testing constant folding.

py/compile.c

@@ -315,7 +315,10 @@ STATIC mp_parse_node_t fold_constants(compiler_t *comp, mp_parse_node_t pn, mp_m
                         pn = mp_parse_node_new_leaf(MP_PARSE_NODE_SMALL_INT, arg);
                     } else if (MP_PARSE_NODE_IS_TOKEN_KIND(pns->nodes[0], MP_TOKEN_OP_MINUS)) {
                         // -int
-                        pn = mp_parse_node_new_leaf(MP_PARSE_NODE_SMALL_INT, -arg);
+                        arg = -arg;
+                        if (MP_SMALL_INT_FITS(arg)) {
+                            pn = mp_parse_node_new_leaf(MP_PARSE_NODE_SMALL_INT, arg);
+                        }
                     } else {
                         assert(MP_PARSE_NODE_IS_TOKEN_KIND(pns->nodes[0], MP_TOKEN_OP_TILDE)); // should be
                         // ~int

tests/basics/int_constfolding.py

@@ -0,0 +1,40 @@
+# tests int constant folding in compiler
+
+# positive
+print(+1)
+print(+100)
+
+# negation
+print(-1)
+print(-(-1))
+print(-0x3fffffff) # 32-bit edge case
+print(-0x3fffffffffffffff) # 64-bit edge case
+print(-(-0x3fffffff - 1)) # 32-bit edge case
+print(-(-0x3fffffffffffffff - 1)) # 64-bit edge case
+
+# 1's complement
+print(~0)
+print(~1)
+print(~-1)
+print(~0x3fffffff) # 32-bit edge case
+print(~0x3fffffffffffffff) # 64-bit edge case
+print(~(-0x3fffffff - 1)) # 32-bit edge case
+print(~(-0x3fffffffffffffff - 1)) # 64-bit edge case
+
+# addition
+print(1 + 2)
+
+# subtraction
+print(1 - 2)
+print(2 - 1)
+
+# multiplication
+print(1 * 2)
+print(123 * 456)
+
+# floor div and modulo
+print(123 // 7, 123 % 7)
+print(-123 // 7, -123 % 7)
+print(123 // -7, 123 % -7)
+print(-123 // -7, -123 % -7)
+

tests/basics/int_divmod.py

@@ -6,12 +6,6 @@ for i in range(-2, 3):
         if j != 0:
             print(i, j, i // j, i % j, divmod(i, j))
 
-# this tests compiler constant folding
-print(123 // 7, 123 % 7)
-print(-123 // 7, -123 % 7)
-print(123 // -7, 123 % -7)
-print(-123 // -7, -123 % -7)
-
 # this tests bignum modulo
 a = 987654321987987987987987987987
 b = 19